The project has two quite distinct objectives, although both employ laboratory procedures and both use the electron microscope extensively: 1. One objective is to elaborate the recent finding in this laboratory that several strains of the amoeba Naegleria gruberi contain an agent that is, in cell-free suspensions, cytopathogenic for cultured chick embryo cells. We now want to investigate how commonly spread is the infectious material among various isolates of this amoeba and, within practical limits, whether other free-living amoebae contain agents pathogenic for mammalian cells. We shall also explore the host range of mammalian cells that are susceptible to cytopathic effects upon inoculation with the agent(s). The infectious material is now known to be serially transmissable in chick embryo cells, thus allowing its characterization to proceed in the same manner as with mammalian viruses. We hope to develop a sensitive and reliable assay for the agent to allow its biological properties to be better characterized. To the degree that it can be purified, its chemical and physical characteristics can be determined by standard procedures. 2. The other objective is to study the structure of viruses, the larger enzymes, and nucleic acids at the electron microscopic level of size. Experiments will be carried on with the in vitro re-assembly of helical structures, such as helical viruses and hopefully, nucleocapsids, and of the smaller isometric viruses, with the objective of learning more of the parameters governing the co-assembly of specific proteins and nucleic acids. The electron microscopic examination of the quaternary structure of oligomeric enzymes will advance an understanding of the principles of assembly of these protein molecules. A particular objective is the improvement of the techniques of electron microscopy, relating to staining methods, characteristics of support films, and effects of electron beam damage.